52 THE HUMAN STERNUM 



presternum and mesosternum, but the hinder end of the sternum is bifurcated, 

 and each bifurcation is again subdivided into long, narrow cartilaginous 

 processes. This condition is directly comparable with the arrangement of 

 the parts in the Ornithodelphia {Ornithorhynchus and £c^z^«a), which cannot 

 but be said to have developed upon parallel lines. 



In the Crocodiles (PL VIII, Fig. 60) there is a further modification of 

 the parts by a simplification of the connexion of the sternum and shoulder- 

 girdle, and its closer association with the ribs. The elements forming the 

 shoulder-girdle are more rudimentary. There are no clavicles. There is 

 a rudimentary interclavicle : and a simple coracoid, articulating with the 

 sternum. The sternum itself is more highly developed. There is an 

 indication of differentiation of presternum, mesosternum, and meta- 

 sternum, but no separation of the several parts has occurred, and the 

 mesosternum is unsegmented. The metasternum is bifid, and may 

 articulate with one or two ribs. The costal connexions of the sternum are 

 much closer than in other reptiles. There are two pairs of cervical ribs, 

 and six, seven, or eight sternal ribs, articulating in pairs with the borders 

 of the sternum. 



The above examples indicate the primary and essential association of 

 the sternum with the shoulder-girdle, and its secondary connexion with the 



ribs. 



D. Birds 



In birds the sternum is broad and simple in character, its chief 

 variability being in the development of the keel. It is characterized by its 

 close and intimate association with the parts of the shoulder-girdle — in which 

 coracoid, precoracoid, clavicles, and interclavicles, are usually present — 

 and by the variability in the number of sternal ribs. Cervical ribs also are 

 commonly present. 



In Rhea Americana there are only three pairs of sternal ribs, crowded 

 together in their articulation with the anterior part ot the lateral border of 

 the sternum. 



